(a) Field of the Invention
The present invention relates to a battery control system for controlling a charged amount of a secondary battery mounted on a hybrid vehicle, and in particular to cancellation of memory effects generated in a secondary battery which uses a nickel compound.
(b) Description of the Related Art
Conventionally, hybrid vehicles which are equipped with an engine-driven generator in addition to a vehicle driving motor have been known. Such a hybrid vehicle is additionally equipped with a battery (a main battery) so that the vehicle driving motor is driven using power from the battery, which is in turn charged using power from the generator. The vehicle driving motor and the engine are used to rotate the wheels for traveling.
A charge level of a battery is observed for control utilizing an index known as the State of Charge, or SOC. The SOC is defined by a ratio of an amount of residual charge relative to a full charge capacity. Presently, SOC is generally measured using combination of a measurement method utilizing correlation between SOC and a battery""s voltage-current characteristics (I-V characteristics) at the time of charging (or discharging), and a measurement method utilizing accumulation of charged and discharged amounts.
A battery control system controls charging and discharging of a battery based on SOC. Specifically, for charging and discharging control, SOC values are divided into three ranges, namely, a charging prohibited range, a discharging prohibited range, and an appropriately charged range. In a charging prohibited range, corresponding to SOC being 80 to 100%, for example, further charging is prohibited because excessive charging could possibly result. In a discharging prohibited range, corresponding to SOC being 0 to 20%, for example, further discharging is prohibited to prevent excessive discharging. In an appropriately charged range, corresponding to SOC being 20 to 80%, for example, excessive charging or discharging are both unlikely, and further charging and discharging are both allowed.
Power consumption efficiency deteriorates in a charging prohibited range because, due to charging being prohibited, redundant energy is not regenerated into power, but is instead discharged as heat or the like. In a discharging prohibited range, on the other hand, a motor is not driven because discharging is prohibited, and engine power is used for charging a battery, which adversely affects a vehicle""s dynamic performance.
In consideration of the above, a battery control system controls such that SOC is maintained within an appropriately charged range. Specifically, driving of a motor and a generator is controlled during ordinary traveling such that an SOC varies within a predetermined control range which is set with a predetermined target charged amount level, for example, about 50%, at the center.
Here, hybrid vehicles often employ NiMH batteries. With batteries using a nickel compound, repetitive charging and discharging within a predetermined control range set around the above mentioned target charged amount level causes memory effects.
Memory effects are caused in electric products by repetitive charging of a battery before full discharging. Once memory effects have been created, an electromotive force of the battery drops after a smaller amount of discharging, even when discharging begins with the battery in fully charged status. That is, the effective operating time of the battery is reduced. Such memory effects are referred to as discharge memory effects for discrimination from the charge memory effects described below. Discharge memory effects can be cancelled by excessive charging of the battery.
Charge memory effects are caused through repetitive discharging of a battery before full charging. Once charge memory effects are caused, the battery""s charge receivability is deteriorated. That is, an electromotive force of the battery reaches a level equivalent to that in a fully charged status when it receives a smaller amount of charge. In still other words, the battery""s maximum charge capacity is reduced.
Whereas charging of ordinary electric devices is basically continued until the device is fully charged, charging of a hybrid vehicle is controlled such that charging and discharging are repeated within a predetermined control range which is set within an appropriately charged range, as described above. Therefore, with a hybrid vehicle, discharge memory effects similar to those in other electrical devices, are caused, and moreover charge memory effects, which are unlikely to be caused in ordinary electrical products, are also likely to be caused.
As described above, a hybrid vehicle uses energy from a battery to enhance dynamic performance, converts kinetic energy into electric energy using a regeneration brake, and stores the converted energy in a battery to improve power consumption. Therefore, when charge memory effects are generated and maximum charge capacity is thus reduced in a hybrid vehicle, vehicle performance is deteriorated because both the amount of energy that can be output and the amount that can be retrieved are reduced.
Such charge memory effects can be countered or cancelled through excessive charging. Japanese Patent Laid-open Publication No. 2001-69608, for example, proposes a technique in which a control range for SOC variation is set larger so as to be closer to a fully discharged or charged level so that memory effects are cancelled.
Although charge memory effects can be cancelled through excessive charging, as described above, excessive discharging generates hydrogen molecules (H2) and oxygen molecules (O2) through electrolysis of water in the battery, which causes internal pressure to increase. Moreover, recombination reaction between H2 and O2 causes temperature to increase. Such increases of internal pressure and temperature accelerate battery deterioration. At present, however, because variation of electric load of a hybrid vehicle during traveling is large, it is difficult to apply stable charging at a low rate so as to prevent increases of temperature and internal pressure for cancellation of charge memory effects.
In addition, because excessive charging for cancellation of discharge memory effects results in significant deterioration of dynamic performance, it is preferable that excessive charging not be applied during traveling.
Further, with the above-noted technique for setting a larger control range for SOC variation so as to be closer to a fully charged or discharged level, a varying SOC remains within an appropriately charged range, which is a middle range where the battery is neither fully charged or discharged, for a longer time. As a result, a longer time is required to cancel memory effects.
The present invention has been conceived in order to address the above problems and aims to provide a battery control system which can relatively smoothly cancel memory effects created in a battery used on a hybrid vehicle, while avoiding deterioration of either vehicle performance or the battery.
According to one aspect of the present invention, there is provided a battery control system comprising memory effects determining means for determining generation of memory effects in the secondary battery and target level setting means for setting the target charged amount level at a predetermined normal level when generation of memory effects is not determined and at a closer-to-limit level closer than the normal level to an upper limit or a lower limit of the appropriately charged range when generation of memory effects is determined.
Specifically, upon determination of generation of memory effects, at least one of these operations is applied, namely, increasing of a target charged amount level from a normal level to the vicinity of the upper limit of an appropriately charged range and reduction of a target charged amount level from a normal level to the vicinity of the lower limit of an appropriately charged range. Increasing of the target charged amount level to a level in the vicinity of the upper limit level can cancel charge memory effects, while reduction of the target charged amount level to a level in the vicinity of the lower limit level can cancel discharge memory effects.
In one embodiment of the present invention, the charged amount may be controlled so as to vary within a predetermined control range which has the target charged amount level at the center and is set within the appropriately charged range, and a control range for the closer-to-limit level may be identical in size to or smaller than a control range for the normal level.
Specifically, a predetermined control range centered at a target charged amount level is set as a target control range of a stored charged amount within the appropriately charged range, and a stored charged amount is controlled so as to vary within this range during most traveling. Because the control range for the closer-to-limit level is either identical in size to or narrower than the control range for the normal level, a control range for a closer-to-limit level is smaller than the appropriately charged range and set closer to either the upper or lower limit of the appropriately charged range. This arrangement increases the likelihood that the charged amount will vary within a range closer to the upper or lower limit, which enables smoother cancellation of charging or discharge memory effects compared to a case wherein, for example, the control range is set as the entire appropriately charged range.
In one embodiment of the present invention, the target level setting means may preferably maintain the target charged amount level at the closer-to-limit level for a predetermined amount of time when generation of memory effects is determined.
Specifically, upon determination of generation of memory effects, the target charged amount level is set at a closer-to-limit level and maintained there for a predetermined time and, after the elapse of the predetermined time, the target charged amount level is returned to a normal level. The predetermined time during which the target charged amount level is maintained at the closer-to-limit level is determined based on the period of time required to cancel memory effects. The period of time required for cancellation of memory effects with the target charged amount level maintained at a closer-to-limit level can vary depending on, for example, a difference between the closer-to-limit level and the upper or lower limit of the appropriately charged range, or the size of a charged amount control range set with the closer-to-limit level at the center.
In another embodiment of the present invention, preferably, the closer-to-limit level may be set in the vicinity of the upper limit of the appropriately charged range.
A secondary battery mounted on a hybrid vehicle is generally a combination battery constituting of a plurality of serially connected batteries (unit cells), in which a charged amount may vary for each unit cell. In this embodiment, by increasing the target charged amount level to be closer to the upper limit of the appropriately charged range, variation in charged amounts among unit cells constituting the secondary battery can be reduced so that charged amounts of the respective unit cells constituting the secondary battery can be made uniform.
According to another aspect of the present invention, there is provided a battery control system for controlling a charged amount of a secondary battery mounted on a hybrid vehicle such that the charged amount is maintained in the vicinity of a predetermined target charged amount level which is set within an appropriately charged range in which excessive charging and discharging are prevented, comprising memory effects determining means for determining generation of charge memory effects and discharge memory effects of the secondary battery; target level setting means for setting the target charged amount level at a predetermined normal level when generation of charge memory effects or discharge memory effects is not determined, in the vicinity of an upper limit of the appropriately charged range when generation of charge memory effects is determined, and in the vicinity of a lower limit of the appropriately charged range when generation of discharge memory effects is determined.